Transactions - move TYPE_SUBSCRIBERS to require optional implementation

Summary: Ref T6403. This one was pretty easy since no one does anything custom with subscribers.

Test Plan: subscribed / unscribed to a random commit ("audit"). joined / left, watched / unwatched a project

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T6403

Differential Revision: https://secure.phabricator.com/D12930